Field devices based upon the capacitive or conductive principle generally have a substantially cylindrical probe unit with at least one probe electrode which can be introduced into the container. For level measurement, rod-shaped probe units extending vertically into the container are known for continuous measurement and, on the other hand, probe units, which can be introduced into the wall of the container, are also known for measuring the level.
In addition, a supplementary electrode, especially a so-called guard electrode, is often used to avoid deposit formation on the probe electrode. The electrode, which coaxially surrounds the probe electrode, is separated from the probe electrode by means of an insulation, and is at the same electrical potential as the probe electrode, as described in, for example, DE 32 12 434 C2. In particular, probe units for point-level measurement are frequently designed in such a way that the probe electrode is at least coaxially surrounded by a guard electrode in the section of the process connection.
The underlying measurement principles of capacitive and conductive level measurement are known from a large number of publications.
In the conductive measuring method, the fill-level is monitored by detecting whether an electrical contact exists between a probe electrode and the wall of a conductive container or a second electrode via a conductive medium. Corresponding field devices are, for example, produced and marketed by the Applicant under the trade name Liquipoint. However, the conductive measuring principle reaches its limits for electrical conductivities ≤0.5 μS/cm, since a change in the conductivity of the medium relative to the conductivity of air is then too small to be reliably detected by the measuring electronics. Media that are difficult to monitor using a conductive measuring method include, for instance, distilled water, molasses, or alcohols. Media with an electrical conductivity of less than 1 μS/cm and a dielectric constant of less than 20 are also problematic. In particular, oils and gases fall into this area.
The capacitive measuring principle is suitable here. The fill-level of the medium in a container is here determined from the capacitance of the capacitor formed by a probe electrode and the wall of the container or a second electrode. Depending upon the conductivity of the medium, either the medium itself or an insulation of the probe electrode forms the dielectric of the capacitor. Field devices based upon the capacitive measuring principle are also produced and marketed by the Applicant in many different configurations, e.g., under the trade names Liquicap or Solicap. Although the fill-level measurement by means of a capacitive measuring method is possible in principle for conductive and non-conductive media, an insulation of the measuring probe is necessary for media with an electrical conductivity >50 μS/cm, which can lead to various disadvantages for the measurement.
Since the advantages and disadvantages of the capacitive and conductive measuring method conflict, a so-called multi-sensor is advantageous, which can determine the fill-level in both a conductive and a capacitive operating mode. By using such a sensor, the level measurement is independent of the electrical properties of the medium. However, various points must be considered when designing such a multi-sensor.
For example, the achievable measuring resolution in the capacitive operating mode is determined by the geometric design of the probe unit. If, for example, the probe unit is designed such that it essentially closes with it after installation in the wall of the container, as in the case of the field device marketed by the Applicant under the trade name FTW33, the measured capacitances can lie in the femtofarad range. If, on the other hand, the measuring probe protrudes at least partially into the container, the measured values for the capacitance are up to several orders of magnitude above this. It goes without saying that the measured capacitances also depend, among other things, upon the medium properties, but these dependencies are to be considered application-specific and, therefore, secondary to the question of the construction of a suitable probe unit.
A further point relates to an electronic unit which is connected to the probe unit and which is used for detection, supply, and/or evaluation of the signal. The components used for this must be adapted to the respective measuring ranges to be expected. This applies, in particular, to the capacitive operating mode, in which capacitances in the femtofarad range have to be detected, which places the highest demands on the electronics unit used.
Finally, a decisive point for the construction of a multi-sensor is given by the contacts of the probe electrode and the additional electrode, as well as their connection to the electronics unit. Cables with strands for electrode contacts are most often used. In doing so, the strands are soldered to the elements to be contacted, while the various cables lead into a common plug. A more elegant method has been disclosed in EP1378014B1 and describes the use of flexible circuit boards for contacting piezoelectric elements arranged in a stack. For this purpose, the flexible printed circuit board has a plurality of contact lugs, which can be glued by bending the piezoelectric elements. This solution is, in particular, more space-saving than the use of cables with strands, but is difficult and complicated in production.
A simpler contacting possibility using a flexible printed circuit board has been disclosed in DE 102011086216A1. A probe unit which is typical of the capacitive and/or conductive mode of operation is described with a contacting module by means of which the probe electrode and the additional electrode are contacted via the flexible printed circuit board. The contacting module has an insulating sleeve and a module housing. The insulating sleeve is designed to receive a section of the rod-shaped probe electrode and to accommodate the flexible printed circuit board. For this purpose, the insulating sleeve has a supporting element, which serves to support two contact regions of the printed circuit board, and which at the same time ensures electrical separation between the two contact regions of the printed circuit board. The module housing holds the assembly together, and, in particular, serves to affix the two contact regions of the flexible printed circuit board. It is made of an electrically insulating material, for example, of a plastic material.
In such a contacting module, the electrode located inside the coaxial structure of the probe unit thus serves as a holder for the contacting module, for which purpose it is not surrounded by the external electrode at least on a section whose length corresponds to the height of the contacting module and a fastening means. The insulation between the probe electrode and the additional electrode can be realized by an element made of an electrically insulating material such as plastic, glass, or else in the form of an air gap.
Since the probe housing serves as a ground potential, however, capacitances between the probe electrode and the probe housing can occur in the region in which the probe electrode is not surrounded by the additional electrode. This leads to interfering influences during the signal transmission from the probe unit to the electronics unit. The shorter the probe unit, the more significant the interfering influences are. In particular, in the case of such probe units, in the installed state essentially complete with the wall of the container as in the case of the aforementioned FTW33, for example, in which the measured capacitances can lie in the femtofarad range, a correct detection of the measured capacitances can be prevented by the interference influences.